The covering power of a silver halide emulsion is a matter of a great concern for emulsion makers, because the amount of silver required to maintain a given optical density can be decreased by using an emulsion having a high covering power. It is disclosed in U.S. Pat. Nos. 4,411,986, 4,434,226, 4,413,053, 4,414,304, 4,414,306, 4,435,501 that the covering power of developed silver can be greatly increased by using tabular grain emulsions having a high aspect ratio (i.e., diameter to thickness ratio) and a small grain thickness.
It is known that the use of tabular grains of a highly sensitive emulsion having a small average grain thickness provides high covering power for silver, as compared with the covering power of emulsions which have a large average grain size and a large average grain thickness.
Moreover, the use of intermediate (from 5:1 to 8:1) and high (more than 8:1) aspect ratio tabular grain emulsions can produce increased image sharpness, and improved speed-granularity relationships. In radiographic elements with emulsion coatings on each of the two opposite faces of the support, marked reductions in crossover have been observed using intermediate and high aspect ratio tabular grain emulsions.
A disadvantage that has been discovered with the use of spectrally sensitized tabular grain silver halide emulsions is related to a bad color tone of developed silver of emulsion grains. The color tone of developed silver of emulsion grains depends upon the grain size and grain thickness. The tone of tabular grains is yellowish, and gives an unfavorable impression to the observer of the resulting picture image. This yellowishness is caused by the strong yellowish light produced by the increase in scattering of blue light due to decrease in size and thickness of the grains. In order to solve such a problem, a tone regulating agent is generally used. For example, a certain kind of mercapto compound may be used for this purpose. However, application of such a conventionally known tone regulating agent to an emulsion having tabular grains causes extreme desensitization, and thus, such an emulsion cannot be put into practice.
Japanese Laid Open Patent No. 158,436/1985 discloses a technique for improving the color tone of a silver image by including a fluorescent brightener in a sensitive material comprising an emulsion of tabular silver halide grains having a grain size of 5 times or more the grain thickness.
U.S. Pat. No. 4,818,675 discloses a technique for improving blackness of a silver image by including a dye having a maximum.. absorption wavelength of from 520 to 580 nm in a sensitive material comprising an emulsion of tabular silver halide grains having a thickness of 0.4 .mu.m or less.
Another disadvantage relates to the formation of dye stain caused by the residual spectral sensitizing dye which remains in the photographic element at the end of the processing. Dye stain can be undesirable in altering image tone. Variations in image tone are particularly undesirable in radiography, since this can complicate proper interpretation of X-ray images. Further, residual dye stain is highly objectionable in that it does not affect all wavelengths equally.
U.S. Pat. No. 4,520,098 discloses a technique for improving dye stain of a silver image by including, in proximity to the spectrally sensitized tabular grains, relatively fine high iodide silver halide grains capable of being dissolved during fixing.